Having a male fussing over not being able to get to a nearby female in heat is much different than simply bringing a bitch in, breeding her, and letting your stud go back to rest. Ejaculation does nothing to diminish strength or energy levels, as EK stated, any tiredness comes from the effort put into getting to the point of ejaculation - which, for dogs (with competent handlers) isn't much.

Ask any teenaged boy or video game nerd that masturbates 70% of their day away, and I guarantee you, the only thing they'll tell you is tired is their arm. Ejaculation has nothing to do with performance.